RDF can be embedded in (X)HTML using simple <script> tags. This is described at http://esw.w3.org/N3inHTML. This gives you a file format that can contain multiple (optionally named) graphs. The document as a whole can "endorse" a graph by including:

<link rel="meta" href="#foo" />

Where "#foo" is a fragment identifier pointing to a graph.

<script type="text/turtle" id="foo"> ... </script>

The rel="meta" stuff is parsed using an RDFa parser, so equivalent RDFa works too.

This module parses HTML files containing graphs like these, and allows you to access them each individually; as a union of all graphs on the page; or as a union of just the endorsed graphs.